Thermal spray coatings for reduced hexavalent and leachable chromuim byproducts

ABSTRACT

A target material for thermal spraying may include chromium and at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese.

TECHNICAL FIELD

The present disclosure relates generally to thermal spray coatings, and more particularly, to thermal spray coatings for reduced hexavalent chromium and leachable chromium byproducts.

BACKGROUND

For decades, chromium plating has been a method of choice when building up worn components and improving wear resistance and corrosion resistance of machine elements. Chromium occurs in the environment primarily in two valence states, trivalent chromium and hexavalent chromium. Of these two states, trivalent chromium is the more stable oxidation state. However, industrial processes such as plating produce hexavalent chromium as byproducts. Hexavalent chromium is a known carcinogen, and emissions of hexavalent chromium are controlled by government regulations. Therefore, in recent years, chromium plating is disfavored because of the hexavalent chromium containing byproducts that are produced during the plating process. To alleviate the problems associated with chromium plating, new coating techniques and materials have been developed that reduce the formation of hexavalent chromium containing compounds during the plating process. Thermal spraying is one such technique that may be used to deposit a coating of a chromium containing material on the surface of a component.

Thermal spraying processes form a coating on a component by melting a consumable material (target material) into droplets and impinging these droplets on the component. Some of the more common thermal spraying processes include flame spraying, plasma arc spraying, electric arc spraying, detonation gun, and high-velocity oxygen fuel (HVOF). The target material (that is, the material to be deposited as the coating) is typically fed into a spray gun as a powder or a wire, where the material may be atomized and accelerated toward the component surface. As the atomized particles impinge upon the surface, they cool and build up into a laminar structure forming the thermal spray coating. In the spray gun, the particles of the target material may be heated to a malleable state before being propelled at high velocity to the component surface. As a heated target material particle (particle) travels from the spray gun to the component surface, some of the chromium in the particles may react with atmospheric oxygen and oxidize to form chromium trioxide (CrO₃) which contains chromium in the hexavalent state. With continued exposure to high temperature, most of this chromium trioxide may reduce to form di-chromium tri-oxide or chromia (Cr₂O₃). However, due to the relatively low vapor pressure of chromium trioxide, some of this material may vaporize off the surface of the particle and cool rapidly in atmospheric air, trapping chromium in the hexavalent state. These escaped particles form the overspray of the plating process. If the amount of hexavalent chromium containing compounds (hexavalent chromium) in the overspray is above acceptable limits, corrective action may be required before the waste may be safely disposed.

In addition to regulations relating to the amount of hexavalent chromium produced in a process, government regulations also regulate the disposal of chromium (and other chemicals) containing waste. Toxicity Characteristic Leaching Procedure (TCLP) is a Federal EPA test method that is used to characterize waste as either hazardous or non-hazardous for the purpose of disposal. The TCLP test simulates landfill conditions, and measures the potential for chromium (or another chemical) to seep or “leach” into groundwater from chromium containing waste potentially disposed in the landfill. While the acceptable TCLP limit for chromium in waste is 5 mg/L or 5 ppm (parts per million), TCLP levels in excess of 100 mg/L have been measured in overspray from plating processes. Therefore, there exists a need to reduce the amount of hexavalent chromium and leachable chromium produced during thermal spraying processes.

U.S. Pat. No. 6,774,076 issued to Yu et al. (the '076 patent) discloses a thermal spray powder to reduce the amount of hexavalent chromium produced in the overspray material. The powder of the '076 patent includes between 45% to 100% by weight of chromia and up to 55% of substantially alpha phase alumina. The alumina of the '076 patent was effective in reducing the amount of hexavalent chromium in the overspray, especially in the substantial absence of alkali and alkaline earth materials that promote the formation of hexavalent chromium. While the thermal spray powder of the '076 patent may reduce the amount of hexavalent chromium is some applications, it may have limitations. For instance, the process of the '076 process may only be applicable when oxides (chromia and alumina) are used as starting materials. Additionally, the presence of alkali and alkaline earth materials in the coating material may reduce the effectiveness of the disclosed powder. Further more, the overspray from the process of the '076 patent may still have unacceptable levels of leachable chromium. The disclosed thermal spray coatings are directed at overcoming these and/or other shortcomings in existing technology.

SUMMARY OF THE INVENTION

In one aspect, a target material for thermal spraying is disclosed. The target material may include chromium and at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese.

In another aspect, a method of applying a coating on a surface using thermal spraying is disclosed. The method includes delivering a target material that includes chromium to a thermal spraying machine. The target material may also include at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese. The method may also include directing particles of the target material to the surface to form the coating.

In yet another aspect, a method of reducing the formation of hexavalent chromium compounds during thermal spraying of a surface is disclosed. The method includes directing particles of a target material from a thermal spraying machine to the surface. The target material may include chromium and at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese. The method may also include forming a shell of at least one of an oxide of aluminum and a chromium oxide manganese oxide spinel over a core of the particles as the particles travel from the thermal spraying machine to the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary thermal spraying system;

FIG. 2 is a schematic illustration of an exemplary sprayed particle of the thermal spray system of FIG. 1; and

FIGS.3A-3B are flowcharts illustrating exemplary thermal spraying processes of the current disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates an HVOF system 20 positioned to deposit a coating 14 on a surface 12 of a component 10. Component 10 may be any component that is desired to be coated with a chromium containing material. For instance, component 10 may be a part of a machine that is being refurbished. In such an embodiment, a surface of the component may be coated with a stainless steel (or other chromium containing material) to restore the original dimensions of the component. Component 10 may also be plastic part that is being coated with a chromium containing material for decorative purposes. Although an HVOF system is depicted in FIG. 1, any thermal spraying technique known in the art may be used to coat component 10.

HVOF system 20 may include a nozzle 28 which is positioned relative to surface 12 of component 10 to deliver a spray 38 of particles 40 to create coating 14 on surface 12. Fuel 22 (such as, for example, kerosene, acetylene, propylene and hydrogen) and oxygen (and/or air) may be fed into a combustion chamber 26 of HVOF system 20. Combustion of the fuel may produce hot, high-pressure gases which may be forced through the nozzle 28. Nozzle 28 may be configured (laval type nozzle, etc.) to increase the velocity of these gases to supersonic velocities and create a jet of hot gases that exit nozzle 28. Target material 30 may be introduced into nozzle 28 as a powder at an entry point 32 downstream of the combustion chamber 24. As target material 30 contacts the jet of hot gases traveling through nozzle 28, the material becomes heated and accelerates to form a stream 38 of particles 40 that are projected at supersonic velocities towards surface 12. Some of these particles 40 may impinge on surface 12 of component 10. The impact of the heated particles 40 upon the surface 12 at supersonic speeds may create a mechanical interlock between surface 12 and particles 40. Over time, the impinging particle spray 38 may produce a uniform coating 14 on surface 12.

Some of particles 40 may not deposit on surface 12 as coating, but may remain in particulate form and/or may settle on surrounding surfaces. These particles 10 that do not form part of coating 14 are generally referred to as overspray 46. The overspray 46 material may eventually be disposed as waste. In some embodiments, a large portion (such as, for example, about 70%) of particles 40 of stream 38 may form overspray 46.

Although FIG. 1 depicts target material 30 as being delivered to HVOF system 20 in a powder configuration, target material 30 may be delivered to HVOF system 20 in any configuration. For instance, in some embodiments, a wire or rod of target material 30 may be introduced into HVOF system 20. Target material 30 may also be introduced at a different location than that illustrated in FIG. 1. For instance, in some embodiments, target material 30 may be fed axially into combustion chamber 26 under high pressure. In practice, the configuration of the target material used, and the method of delivery of the target material, may depend upon the type of thermal spraying technique employed.

Target material 30 may include any type of chromium containing material that includes aluminum between about 0.5% and about 12% by weight, and/or manganese between about 2% and about 15% by weight. In some embodiments, both manganese and aluminum may be present, while in others only one of aluminum and manganese may be present. The choice of aluminum or manganese, and the proportions of aluminum and/or manganese in target material 30 may depend upon the proportions of the other constituents of target material 30, and the manufacturing process of target material 30. For instance, if the manufacturing process of the target material 30 includes heating (such as, during sintering) target material 30 to a temperature above (or close to) the melting temperature of aluminum, then target material 30 may include manganese in place of aluminum.

In a thermal spraying application where a chromium carbide-nickel chromium material is to be coated on surface 12 of component 10 as a chromium replacement coating, target material 30 may be a chromium carbide-nickel chromium material with aluminum between about 0.5% and about 12% by weight, and/or manganese between about 2% and about 15% by weight. Target material 30 may include any composition of chromium carbide and nickel chromium that is used in the art. As mentioned earlier, the choice of aluminum or manganese, and the percentage of aluminum and manganese in target material 30 may depend upon the composition of chromium carbide and nickel chromium in the material. In an application where the chromium carbide-nickel chromium target material includes up to about 80% by weight of chromium carbide, and up to about 20% by weight of nickel chromium, target material 30 may include between about 4-8% by weight of manganese.

In another embodiment of chromium replacement coating, target material 30 may include about 9-12% by weight of carbon, about 15-20% by weight of nickel, less than about 0.5% by weight of iron, about 2-15% by weight of manganese, and the remaining chromium. In some applications, the manganese in target material 30 may be replaced with (or supplemented with) about 0.5-12% by weight of aluminum. In yet another embodiment of a chromium replacement coating, target material 30 may be an iron containing material that includes about 20-80% by weight of chromium, about 10-75% by weight of iron, about 5-10% by weight of carbon, 0.5-30% nickel, and one or both of about 2-15% by weight of manganese, and/or about 0.5-12% by weight of aluminum. In some embodiments of the chromium replacement coating, a desired percentage of manganese and aluminum may be between about 3-9% by weight and about 6-12% by weight respectively. In some other embodiments of the chromium replacement coating, a desired percentage of manganese and aluminum may be between about 5-7% by weight and about 9-11% by weight respectively.

In an application where thermal spraying is used to refurbish components, target material 30 may include an alloy of stainless steel (such as 420 stainless steel) and one or both of about 2-15% by weight of manganese and about 0.5-12% by weight of aluminum. In a refurbishing application, target material 30 may include less than about 0.4% by weight of carbon, about 12-14% by weight of chromium, less than or equal to about 1% by weight of silicon, traces (≦about 0.15% by weight) of phosphorous and sulfur, and one or both of about 2-15% by weight of manganese and about 0.5-12% by weight of aluminum. In some embodiments of the chromium replacement coating, a desired percentage of manganese and aluminum may be between about 3-9% by weight and about 6-12% by weight respectively. In some other embodiments of the chromium replacement coating, a desired percentage of manganese and aluminum may be between about 5-7% by weight and about 9-11% by weight respectively.

The embodiments of target material 30 illustrated in the previous paragraphs are exemplary only. In general, target material 30 may be any chromium containing material having one or both of about 2-15% by weight of manganese and about 0.5-12% by weight of aluminum. Although, in general, any amount of chromium may be present in target material 30, for most benefit, the amount of chromium in target material 30 may be greater than about 1% by weight. In some embodiments, target material 30 may be an alloy of different materials including chromium. In other embodiments, target material 30 may include a core of one material and one or more shells (or layers) made of other materials. In an embodiment where target material 30 is in the form of a wire, the wire may include a core made of one material and one or more shells of different materials. In such layered embodiments, the different materials of target material 30 may form molten particles 40 that impinge on surface 12 to mix and create coating 14.

FIG. 2 is a schematic illustration of a particle 40. Particle 40 may include a core 42 and a shell 44 around core 42. Core 42 may be made of substantially the same material as target material 30, while shell 44 may be made of an oxide of aluminum (such as, alumina Al₂O₃) or an oxide including manganese, such as chromium oxide manganese oxide spinel (chrome manganese spinel). In an embodiment of target material 30 than includes aluminum, shell 44 may be alumina, and in an embodiment of target material 30 which includes manganese and not alumina, shell 44 may be made of chrome manganese spinel. In embodiments of target material that include both aluminum and manganese, shell 44 may include both alumina and chrome manganese spinel, the relative proportions of which may depend upon the percentage of aluminum and manganese in target material 30. Although FIG. 2 illustrates a continuous shell 44 that forms all around core 42, it is contemplated that in some embodiments, shell 44 may be discontinuous and include gaps that expose portions of core 42.

In applications with a shell 44 made of alumina, the alumina shell may protect the material of the core 42 from oxidation and reduce (or prevent) the formation of chromium trioxide. Since the kinetics of the reaction producing alumina are very high, shell 44 may be formed before any significant amount of chromium in core 42 oxidizes to form chromium trioxide. This reduction (or prevention) in the formation of chromium trioxide by shell 44 may reduce the amount of hexavalent chromium produced in the plating process.

In applications with a shell 44 made of chrome manganese spinel, some of the chromium in core 42 may react with oxygen in the atmosphere to form chromium trioxide. However, before the chromium trioxide can vaporize off particle 40 and cool down, trapping chromium in the hexavalent state, the manganese in core 42 may react with the chromium trioxide to form chrome manganese spinel. The high vapor pressure and high chemical stability of chrome manganese spinel may substantially reduce further oxidation of core 42 and the formation of hexavalent chromium.

In particles 40 of overspray 46 disposed in a landfill as waste, shell 44 may trap the chromium in the core 42 and reduce (or prevent) the chromium from leaching out of the core 42. Although the disclosure discussed the reduction of leachable chromium from overspray 46, it is contemplated that shell 44 may also reduce the leaching of other compound (such as nickel, and cobalt) from over spray 46. Thus, the alumina or chrome manganese spinel shell 44 of particles 40 may also reduce the TCLP of chromium and other compounds in the waste produced by the thermal spraying process.

Industrial Applicability

The disclosed thermal spray coatings may reduce the amount of hexavalent chromium, leachable chromium and other compounds produced as byproducts of the plating process. Manganese and/or aluminum may be added to a chromium containing target material that is to be thermally plated on a surface. The manganese and/or aluminum may produce a chemically stable oxide shell around the sprayed particle. The stable oxide shell protects the core of the particle from further oxidation, thereby protecting the chromium and preventing the formation of hexavalent chromium. The chemically stable shell around the core may also prevent leaching of chromium and other compounds from the core, and thereby reduce the amount of leachable chromium and other leachable compounds produced in the thermal spray process.

FIG. 3A and 3B illustrate methods of thermal spraying a surface 12 to create coating 14, and reducing hexavalent chromium and leachable chromium produced in the process. FIG. 3A illustrates a thermal spraying process that uses aluminum in the target material, while FIG. 3B illustrates a process that uses manganese in the target material. Referring to FIG. 3A, a target material that includes chromium, and between about 0.5-12% by weight of aluminum may be directed to a thermal spraying machine (step 110A). In some embodiments of the process, the percentage of aluminum in the target material that is directed to the thermal spraying machine may be between about 9-11% by weight. In the thermal spraying machine, the target material may be transformed into heated particles that are directed to surface 12 (step 120A). A shell 44 of alumina may be created over a core 42 of the target material as the particles 40 proceed to surface 12 (step 130A). Shell 44 may prevent the formation of hexavalent chromium and leachable chromium containing compounds in the method illustrated in FIG. 3A. A portion of particles 40 may deposit on the surface 12 and create coating 14 (step 140A), while another portion may form overspray 46.

In the method illustrated by FIG. 3B, a target material that includes chromium, and between about 2-15% by weight of manganese may be directed to a thermal spraying machine (step 110B). In some embodiments, the percentage of manganese in the target material may be between about 5-7% by weight. As in the method illustrated by FIG. 3A, the target material may be transformed into heated particles 40 of the target material in the thermal spraying machine (step 120B). These heated particles 40 may be directed to the surface 12 that is to be coated. As the particles 40 travel to surface 12, a shell 44 of chrome manganese spinel may be formed over the surface of particles 40 (step 130B). This chrome manganese spinel shell 44 may reduce the formation of hexavalent chromium and leachable chromium containing compounds in the method of FIG. 3B. The particles 40 may then proceed to surface 12 where a portion of the particles 40 may get deposited to form coating 14 and the remaining portion may form overspray 46.

Since the oxide shell reduces both hexavalent chromium and leachable chromium containing compounds, use of the thermal spray coatings of the current disclosure may eliminate issues related to both hexavalent chromium and leachable chromium in overspray 46. Additionally, since the kinetics of formation of the alumina shell and the chrome manganese spinel shell are relatively unaffected by the constituents of the target material, the disclosed thermal spray coatings may be effective for a wide range of target materials.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed thermal spray coatings. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed thermal spray coatings. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents. 

1. A target material for thermal spraying, comprising: chromium; and at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese.
 2. The target material of claim 1, wherein the target material includes manganese.
 3. The target material of claim 2, wherein the target material includes manganese between about 5-7% by weight.
 4. The target material of claim 1, wherein the target material includes aluminum.
 5. The target material of claim 4, wherein the target material includes aluminum between about 9-11% by weight.
 6. The target material of claim 1, further including up to about 80% by weight of chromium carbide and up to about 20% by weight of nickel chromium.
 7. The target material of claim 1, further including about 9-12% by weight of carbon, about 15-20% by weight of nickel, and less than about 0.5% by weight of iron.
 8. The target material of claim 1, further including a iron containing material.
 9. The target material of claim 8, wherein the iron containing material includes about 20-80% by weight of chromium, about 10-75% by weight of iron, about 0.5-30% by weight of nickel, and about 5-10% by weight of carbon.
 10. The target material of claim 1, further including less than about 0.4% by weight of carbon, about 12-14% by weight of chromium, less than or equal to about 1% by weight of silicon, and less than or equal to about 0.15% by weight of phosphorous and sulfur.
 11. A method of applying a coating on a surface using thermal spraying, comprising: delivering a target material that includes chromium to a thermal spraying machine, the target material also including at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese; and directing particles of the target material to the surface to form the coating.
 12. The method of claim 11, further including forming a shell of one or more of an oxide of aluminum and a chromium oxide manganese oxide spinel over a surface of the particles as the particles are directed from the thermal spraying machine to the surface.
 13. The method of claim 11, wherein delivering the target material includes delivering a target material that includes about 0.5-12% by weight of aluminum.
 14. The method of claim 13, wherein delivering the target material includes delivering a target material that includes about 9-11% by weight of aluminum.
 15. The method of claim 11, wherein delivering the target material includes delivering a target material that includes about 2-15% by weight of manganese.
 16. The method of claim 15, wherein delivering the target material includes delivering a target material that includes about 5-7% by weight of manganese.
 17. A method of reducing the formation of hexavalent chromium compounds during thermal spraying of a surface, comprising: directing particles of a target material from a thermal spraying machine to the surface, the target material including chromium and at least one of about 0.5-12% by weight of aluminum and about 2-15% by weight of manganese; and forming a shell of at least one of an oxide of aluminum and a chromium oxide manganese oxide spinel over a core of the particles as the particles travel from the thermal spraying machine to the surface.
 18. The method of claim 17, wherein the target material includes at least one of about 9-11% by weight of aluminum and about 5-7% by weight of manganese.
 19. The method of claim 17, wherein the target material is a chromium replacement coating material.
 20. The method of claim 17, wherein the target material includes a stainless steel alloy. 